Who Made That Captcha?

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By Daniel Engber

Jan. 17, 2014

Before the presidential election in 1996, a computer company called Digital Equipment Corporation set up a website for opinion polls. There was one problem, though: Partisan hackers could use bots to fill the site with useless spam. So D.E.C. put a pixelated image of the U.S. flag at a random spot on the screen that users had to click before they registered their votes.

The flag didn’t work too well — simple programs could be written to defeat it — but the basic idea would soon be copied and expanded. In September of that year, Moni Naor, a computer scientist at Israel’s Weizmann Institute, decided that what was needed was something like a Turing test — in which a human tries to figure out if he’s talking to a computer — but in reverse. A computer would have to figure out if it was talking to a human, using a test that would be easy for people to pass but impossible for machines. What sort of test might work? Naor’s suggestions didn’t seem so practical: A user might be shown a portrait, he said, and asked to name its subject’s sex. Or he might be presented with an image of several people and told to find the one who wasn’t wearing any clothes.

A team of scientists at D.E.C., led by Andrei Broder, soon devised a better solution. They put a string of characters on the screen, masked with gridded lines or other noise, and then asked the user to type the characters in a space below. “Broder’s system was pretty breakable by anyone who knew a little bit about machine vision,” says John Langford, a Microsoft researcher, referring to the field of study that brought us digitized books. “But it was enough to deter people.”

A few years later, a cryptographer at Carnegie Mellon, Manuel Blum, perfected the idea. With the help of several graduate students, including Langford and Luis von Ahn, Blum set up the first effective system for screening out spam bots: strings of letters and numbers that were distorted using simple software, random numbers and a standard set of rules. “The program was completely public,” Blum says. “Except for those random numbers, everyone would know exactly how the program worked.”

What about the name Captcha? That was Blum’s idea. “I remember trying to come up with a ‘gotcha’-like acronym.” The best he could manage was: Completely Automated Public Turing test for telling Computers and Humans Apart. Another researcher working on the same topic wanted to call the system a Reverse Turing test. But when Yahoo started using the Carnegie Mellon system, the company described it as a Captcha, and the name stuck.

CAPTCHA OF THE MIND

Thomas Hannagan is a cognitive scientist at the French National Center for Scientific Research and Aix-Marseille University.

Why is a cognitive scientist studying Captcha? These images tell us something about visual word recognition. We’ve been working on how humans recognize words for more than 100 years, but we still can’t fix that system when it’s broken.

You showed distorted words to subjects for fractions of a second. What did that tell you? A very rapid mechanism abstracts information from the letters. It’s not dependent on what color it was or the shape of the letters.

Do you have more Captcha experiments in mind? In our lab, people have been working with baboons. They have almost the same visual system as ours, but without any language. If a baboon could process a Captcha, it would tell us that this ability comes, not from language, but from the visual system.

You don’t really think a baboon could solve a Captcha? That’s my next experiment!